1. Field of the Invention
The invention relates to a frequency converter circuits for use with wind power generating equipment and, more particularly, to such a circuit for a double-fed asynchronous generator with a variable power output, which can be connected to a voltage network, and a method for operating such a frequency converter circuit.
2. Description of the Related Art
Asynchronous generators with variable power output are widely used today in the field of energy generation. In particular, they are employed in connection with wind power generators, whose power output naturally greatly varies with the prevailing wind. A variable operating speed is of great advantage in connection with such asynchronous generators. A wind power generator with variable speed does permit better utilization of weak winds and exerts less mechanical stress on the tower of the wind power generator in strong winds.
A frequency converter circuit is required between an asynchronous generator and the voltage network for operating the former, into which the asynchronous generator is intended to feed energy. Frequency converter circuits for double-fed asynchronous generators in windmills are widely known and, as a rule, have a three-phase current frequency converter at the rotor side, intermediate circuit capacitors and a frequency converter on the network side. Such a classic frequency converter circuit for a double-fed asynchronous generator provides the two above mentioned advantages, but is comparatively expensive.
A simple solution regarding the double-fed asynchronous generator is known from U.S. Pat. No. 7,015,595, which offers a speed regulation in the sub-synchronous range. In this case “sub-/super-synchronous” refers to the so-called synchronous number of revolutions, or nominal rated speed of the asynchronous generator and means a number of rotor revolutions smaller/greater than the synchronous number of revolutions. The energy yield at low wind speeds is increased because of this. However, in case of a sudden increase in the wind speed, the windmill tower is not relieved.
A solution for windmills with an asynchronous generator with a short-circuited rotor and power electronic drive for smaller outputs and with an improved energy yield at low wind speeds is known from U.S. Pat. No. 7,012,409. Here, too, the windmill tower is not relieved in case of strong increases in wind speed.
An alternative option for tower relief is known from the prior art. Here, a resistor, which is switched on in case of higher wind speeds, is been inserted into the rotor circuit. Then the rotor current flows through the resistor. By means of this, the rotational speed of the windmill can be increased, additional wind energy is stored as additional kinetic energy of the windmill and the tower is mechanically relieved. Since the tower is mechanically relieved, the tower structure can be provided in a simpler and more cost-effective way. The “optislip” circuit with a resistor is completely retained in the rotor, or rotates along with it. Therefore no contact rings are provided. The rotor energy additionally heats the resistor, and therefore the rotor, and thus the generator. In actual use, the super-synchronous operation is briefly limited here to a few percent.